It will not work as dual channel, OR, on some boards, 2GB from each module may work as dual channel while the remaining 6GB on the larger module work as single channel, but there are few boards that I'm aware of that will work in this way.
It's also possible that they may not want to work together at all if the specs are much different or even if they are exactly the same specs except for the size of the memory module.
It is always, in ALL cases, better to use two identical sticks that came together in a set and were tested at the factory to be compatible with each other. You will have much better chances of success using two or more sticks, if they came together in a set.
As for the single 8GB stick, if you do not plan to add another 8GB stick later, why not just go with two 4GB sticks? It would be a better configuration.
If you must use a single stick though, I will need to know where you can purchase memory from in order to recommend products that are available to you, rather than ones that you may not have access to as in other countries.